Location of supply ducts (ceiling or floor) depends on whether it’s a heating or cooling climate
Can someone confirm what I’ve been told about locating the supply registers? We’re going to build in the North Atlanta metro area in zone3, primarily heating climate. I was told that in a primarily heating climate, the best location of the supply registers is close to the floor. While in a primarily cooling climate, the best location is the ceiling.
I’m asking since we plan on building on a conditioned crawlspace and I plan on locating the mechanical there. So easy access to the main floor and I don’t need to plan for any duct chases up to the attic and any distribution ducting up there. I was planning on doing a conditioned attic but I want the option of doing a vented attic to lower the cost of the insulation up there.
PS, this is a single story modern Farmhouse design with a central vaulted area along with side sheds. No easy connection between the attic area’s above the side sheds. I do plan on having small duct chases up to the attic to run the ventilation ducts.
Anyways, I’ve not really found anything that confirms what I’ve been told about location of the supply registers based on heating/cooling climate.
Thanks.
George Wise
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Replies
https://www.greenbuildingadvisor.com/question/forced-air-heat-register-location
With good windows and very good air sealing the issue isn't so much as which is better for heating/cooling climates but finding the most efficient design for the duct system as a whole.
IMO supply in the ceiling allow for easier configuration of furniture especially in bedrooms.
In a high-R house the location of the registers hardly matter. Even for a well though out code-min house it's not a big deal.
Keeping the duct runs inside of conditioned space IS big deal though. While the old-schoolers typically put the registers above or below a window, installing ceiling (or wall) register next to (or inside) an interior wall with diffusers that direct the air toward an exterior wall works. Even code-min windows are sufficiently better than the leaky single panes of yore that the convection at cold/warm window just doesn't matter as much as it used to.
Dettson's mini-ducted small gas furnace + heat pump solutions use ducts small enough to fit in holes bored through the framing non-load-bearing 2x4 walls, minimizing the amount of any soffitted duct run needed below the main ceiling level.
Using a plenum truss for the attic/roof would also allow free space for duct runs:
https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic
Supply registers just need to create good mixing (per Manual T). Return location (high or low) has an effect on heat pump efficiency (even proper designs can stratify up to 3C). Maybe a 20% efficiency hit at 3C and 10% at a more likely 1.5C.
Adding a nice looking prop fan can greatly supplement the central air system, and the placement of the supply grilles becomes a secondary issue. The prop fan can be controlled based on the usage of the space and on the season you are in (upwards or downwards flow)
this dates back to putting radiators below windows...it certainly became less important when people went to central air systems, which typically cycled on and off during the day. (what good is it having a register blowing warm air in front of a window when half the time it's not blowing). As architects (not green building nerds), we put registers where on balance they work best after consideration or where ducts can run, furniture placement, etc. One favorite spot for us in bedrooms for either mini-split heads or registers is above the bed...this might sound counter intuitive, but there's usually enough velocity to "throw" over the head of the bed, much better than a ceiling or wall location that blows at the bed.
> radiators below windows
More significantly, better windows have reduced the need. But room-side low-E coatings bring some of the downdraft problem back. Either way, the effect is still there - at 0F and 3', more than 30% of people will be uncomfortable. Variable speed central air systems + properly designed under-window registers should be able to provide reliable close-to-window comfort.